(1) Field of the Invention
The present invention relates to a discharge type transfer device for transferring the developer to the paper so as to reproduce an image from an original, in particular relating to a transfer device which provides unvarying transfer performance regardless of the type of sheet, by perceiving the difference of sheet rigidity.
(2) Description of the Prior Art
Some transfer devices for use in electrophotographic image forming apparatuses such as copiers, printers and the like apply a transfer voltage by means of a transfer roller or transfer belt in order to transfer the developer (toner etc.) image onto an electrostatic latent image support (photoreceptor). Control of this transfer function is performed based on a constant-current system and is adapted to deal with change in transfer conditions within limits. For example, the control is made in accordance with change in the environmental conditions (high-temperature high-humidity, low-temperature low-humidity) of the image forming apparatus. However, the types of sheets used by the transfer device are diverse. For example, sheets with a paper weight of 50 g/m2 to 250 g/m2 may be handled.
Control of the transfer current in accordance with each sheet type is difficult to make by the above control, which results in causes of print failures and machine troubles.
To deal with this problem, some techniques for varying the transfer voltage applied to the transfer roller or transfer belt, depending on the environmental change of the image forming apparatus and the sheet type, based on the measurement of the transfer current value, have been proposed (see, for example, a patent publication 1: Japanese Patent Application Laid-open Hei 06-308844 ([0015] to [0018] on page 4, [0075] on page 9 and FIG. 1).
Other than the above, transfer devices using a corona discharger have been disclosed (see, for example, a patent publication 2: Japanese Patent Application Laid-open Hei 04-171463).
In the technique disclosed in the patent publication 1, the recording medium is fed into the transfer station where the transfer element is put in contact with the image support and the resistance at different points of the recording medium is determined at every predetermined interval based on the transfer current flowing through the transfer element, so that the transfer voltage is changed based on the result. Therefore, the transfer condition may change at the front part and the rear part over one sheet, causing difficulties in securing even printing quality. Further, since in the above publication technique, the transfer current needs to be measured at every predetermined interval so as to determine the transfer voltage based on the data, it is necessary to provide transfer tables for various environmental conditions in the controller of the image forming apparatus.
Further, for the recording media such as OHP sheets and the like, which have a volume or surface resistivity greatly different from usual, a recording media detecting means is needed so as to determine the type and set up different transfer conditions in accordance with the judgement. Thus, in order to make control involving the measurement of environment, the controller needs to implement markedly complicated control and store and handle a huge amount of data.